1. Field of the Invention
The present invention relates to an unsaturated polyester resin system which is a high performance electrical insulation material useful in the electronic and microwave industries, and a process for preparing such resins.
2. Technology Review
Electrical circuit boards are prepared by laminating sheets of electrical conducting material, such as (but not limited to) copper, onto a base or support of insulation material. The reliability of performance for the finished circuit board depends to a great degree on the physical, mechanical and electrical characteristics of the support material itself. Ideally, this support material would be chemically resistant to acids and possess a high Tg, a low dielectric constant, a high comparative tracking index, a low coefficient of thermal expansion, high surface and volume resistivity and could additionally be processed with conventional manufacturing procedures.
In general, electrical insulation materials many times contain a structural reinforcement, such as glass fibers, to improve the material strength of the product. Depending on the particular resin and intended use of the board, these reinforcements may be either woven or non-woven, and may be dispersed throughout the resin in either a randomly oriented or non-randomly oriented fashion. For example, an electrical-grade glass mat sheet is representative of a woven, non-random support, whereas randomly distributed, chopped glass fiber is an example of a non-woven, randomly oriented support. Standards for polyester glass-mat sheets have been established by the National Electrical Manufacturer's Association. Both organic and inorganic materials are suitable as a structural reinforcement. Additionally, other fillers may be incorporated in the support material.
Electrical insulation materials, especially those in circuit boards, also frequently contain flame retardants. These flame-retardant may be chemicals that can be incorporated in the form of a monomeric unit during the curing of the resin. Alternatively, flame-retardant monomers may be incorporated in the production of the base resin. Examples of monomeric flame retardant materials are brominated, chlorinated or other halogenated vinyl monomers. For example, Dibromostyrene (DBS), available from Great Lakes Chemicals, El Derado, Ark., is a suitable brominated vinyl monomer. The flame retardant materials may also either be organic, as in the case of the above-mentioned monomers, or may be inorganic. Aluminum trihydrate is representative of an inorganic flame retardant suitable for use in electrical circuit board supports.
Of the physical characteristics upon which circuit board reliability depends, one of the most critical is the magnitude of z-axis expansion during thermal cycling. The electrical insulation materials which possess a high coefficient of thermal expansion expand to a much greater degree than the metal hole barrels, resulting in the deformation of the barrels and subsequently a substantial number of failures in the circuit boards. This failure due to high z-axis expansion is magnified in double-sided boards due to the high concentration of resin in these boards. Generally, the number of failures can be expected to increase with increasing number of thermal shock cycles, but the number of failures may be reduced somewhat by a prebake of the material.
An additional problem associated with certain electrical insulation materials is that the z-axis expansion increases at a much higher rate at temperatures above the Tg than at temperatures below the Tg. Such a characteristic of the electrical insulation material may exacerbate the aforementioned problem of metal plated through hole failures. As a result, electrical insulation materials that have both a relatively high Tg and a relatively low rate of change in z-axis expansion above the Tg would be preferred. Thus, although it is difficult to greatly change the expansion properties of a material, much effort has been placed in achieving even modest improvements in this property.
There are two general types of electrical insulation support materials for high performance uses currently employed in the electronic circuit board industry: those based on epoxy resins and those based on much more expensive materials. The epoxy electrical insulation resins, exemplified by FR4 resin, have the particular advantages of both ease of processing and low cost of manufacture. For example, the manufacture of laminates comprising epoxy resins can be highly mechanized. Along with the low cost of base materials, this mechanization results in superior production costs as compared to other generally used electrical insulation support materials. The highly favorable cost and processing advantages of the epoxy resins are substantially offset, however, by the physical and electrical properties of these materials. For example, FR4 epoxy resin has a z-axis expansion of 55 to 80 ppm/.degree.C. below Tg and of 250 to 400 ppm/.degree.C. above Tg. This expansion corresponds to about 2.2% to 3.0% between 40.degree. C. and 180.degree. C., and about 4.4% between 20.degree. C. and 260.degree. C. Such expansion properties do not compare well with that of copper, which expands at about 18 ppm/.degree. C. Additionally, FR4 possesses a relatively low Tg of about 120.degree. C. The poor electrical properties of FR4 and similar resins include a relatively high dielectric constant of about 4.8, a relatively high dissipation factor of about 0,020, a relatively low minimum surface resistance of 5.times.10.sup.3 megohms and a relatively low volume resistivity of 1.times.10.sup.5 megohm-cm. As is known to one skilled in the art of electrical insulation materials, the aforementioned relatively poor physical and electrical properties of epoxy resin render this material unsuitable for use as an electrical insulation support in demanding applications, particularly those applications involving high temperature manipulations or operations.
The second general class of electrical insulation materials, the "high performance" insulation materials, overall have physical and electrical properties superior to those of epoxy resin systems. The high performance resins generally possess a relatively low z-axis expansion, a relatively high Tg, a relatively low dielectric constant, and relatively high surface and volume resistivity. Examples of high performance electrical insulation materials are those made of polyimide, cyanate ester and PTFE.TM. (Dupont). By "high performance electrical insulation material" it is meant that the electrical insulation material has a plurality of physical and electrical characteristics which render the resin superior to that of a traditional epoxy resin in electrical and related applications. For example, polyimide resins can withstand repeated exposure to temperatures up to 260.degree. C., the temperature of a liquid solder bath, with generally a lower number of failures in metal plated through holes due to temperature stress. This is because, for example, polyimide possesses a relatively high Tg of 205.degree. C., and a z-axis expansion below Tg of 49 ppm/.degree.C., and that above the Tg of 195 ppm/.degree.C. This expansion corresponds to about 0.7% between 40.degree. C. and 180.degree. C. The electrical properties of the high performance electrical insulation materials are also generally superior to those comprising epoxy resins, although expensive enhancement to the epoxy can bridge this gap. As an example of the superior electrical properties of high performance materials, polyimide has a relatively low dissipation factor at 1 MHz of 0.011.
The primary drawbacks associated with the high performance electrical insulation materials is that the cost of laminates comprising these materials is prohibitively high for general use in the electronics industry. For example, in addition to the relatively high cost of base materials, the high performance electrical insulation materials are very difficult to process, resulting in low yields of the final product. Additionally, the dimensional stability of the high performance materials is much less than that of even FR4 during processing, since the shrinkage is not radial as in the epoxy resins, but rather uni-directional. The high performance electrical insulation materials are also inert to most available conditioning processes for the through hole wall, which contributes substantially to the number of plated metal through hole failures.
Resins consisting of unsaturated polyester alone have not been generally useful as an electrical insulation material for several reasons including excessive brittleness and unacceptable levels of z-axis expansion. In the absence of a large amount of monomer, a completely unsaturated polyester resin is highly reactive, resulting in a very brittle resin. This brittleness makes processing these resins very difficult, particularly when thinness is desired as, for example, in use as an electrical circuit board support. Additionally, any cracks that develop in products made with these resins subsequently propagate throughout the product. In the presence of the amount of monomer needed to overcome the brittleness of unsaturated polyester resins, the resins then becomes too flexible and expandable, with the result that the z-axis expansion becomes unacceptable for applications or processes in which the temperatures encountered vary greatly or are extremely high. For example, similar to the epoxy resins, under the high temperature conditions encountered in the processing of electrical circuit boards, an unsaturated polyester support leads to failure in a substantial number of the metal plated through holes. A further potential drawback to the use of unsaturated polyester resins is that the conducting metal foil of an electrical circuit board does not generally bind strongly to the resin. Use of adhesion promoters will, however, substantially overcome this bond strength problem. For example, U.S. Pat. No. 4,093,768 discloses that when the adhesion promoter benzotriazole is incorporated into an unsaturated polyester resin at up to about 2% by weight, then a copper foil can be directly pressure bonded to the resin. However, due to limitations such as those described above, it was not possible to make reliable materials for general use and with acceptable electrical insulation properties from unsaturated polyester resins. The same disadvantages associated with the polyester resins and epoxy resins, also hold true for mixed polyester/epoxy resins.
As a result of the various advantages and disadvantages associated with the known electrical insulation materials, one in the art must determine which type of material to employ based upon the specific final application. Particularly when cost is a predominant factor in this determination, one is constrained to the use of the lower performance materials of the epoxy or similar type. In other cases, one is confined to use the high performance materials, regardless of the cost, to satisfy the requirements of the particular processing or final application.
There thus exists a great need for discovering materials which are suitable for general use as an electrical insulation support and which have both the excellent physical and electrical properties of the high performance materials as well as the low cost and ease of processing of the epoxy and similar type resins.